I recently completed a survey to determine the energy efficiency of my home. The results weren't great but got me thinking about the quick and affordable things I could do to improve things and hopefully reduce our outgoings on energy.

One thing that has come up a few times is the use of radiator reflectors. Energy companies recommend them and the UK Government has approved two for use, but I am a bit sceptical as to how good they are.

really interesting question - I have been checking out online info sources and it seems there is very little to support my initial thought that they would be effective. It seems a much more important requirement is increasing airflow and turbulence past the radiators.
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Rory AlsopMar 23 '12 at 13:58

Either you read a different Wikipedia article than you linked, or it's updated since you asked. That article indicates pretty much exactly what I expected: most heat produced by radiators is air currents, and thus a reflector is only as good as the degree to which it improves the insulation rating of the wall. In an old wall, that could have some effect, but not so much with modern insulation methods.
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ScivitriMar 23 '12 at 15:36

That's pretty much what it said when I looked, but I wasn't sure if my interpretation was correct. I was hoping someone here may have tried them and could share their experience/thoughts.
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cchanaMar 25 '12 at 8:47

2

The folks at Skeptics might be able to dig up some research/studies on the subject.
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Tester101♦Mar 26 '12 at 18:28

Thanks for the suggestion, I have posted it there now too.
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cchanaMar 29 '12 at 10:28

2 Answers
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These devices are snake oil and would not be used even in Scandinavia where we know a good deal about insulation. Radiator heating is not based on radiation but air circulation and radiation to the back wall has practically a zero effect on energy loss.

Most of the gear marketed as "energy savers" is bluff. Even the online calculators of the window companies show that switching from double-glass windows to triple glass ones will never pay off. So just think about a small vertical patch pretty close to the floor. Heated air always goes up and it is always advisable to add more insulation above the ceiling (in case there is a cold loft above it). That is where the heat gets out of the room in 99% of the cases - provided your windows are draft-proof.

A few years ago I lived in Scotlad and had triple glass windows. Gas consumption was very high and the rooms still cold. Well, one day I could hear crows jumping on the roof... presumably there was no insulation at all above the ceiling. After some calculations I could figure out that more energy was consumed per square meter in Scotland than in wintry Scandinavia where the temperature was 20C lower.

As it turns out, the Wikipedia article itself actually points to some (albeit Radflex funded) reports. The second report BRE shows ~45% reduction in heat lost to the wall behind the radiator.

Tables 4 to 11 of the BRE report show a range of figures to do with efficiency and energy saving. Table 4 shows the amount of energy you could save on an annual basis, broken down by the energy loss (insulation efficiency) of the wall:

Assuming an area of 1 meter per radiator and 10¢ per kWh (rough average of US price), you this will save $2-$9/year for each radiator. However, these are all back-of-the-envelope calculations based on a study paid for by Radflek and they don't even have full distribution across the EU (let alone the US)5. AFAIK, the heating hours are calculations done for climate emissions ratings, so it's a big average of heating of UK(?) homes. Will a competitor be as efficient? Is your wall going to leak heat the same as an averaging of a bunch of houses in another area? Who knows!

So is it worth the $29 (£18) price for the minimum order of 6 reflectors to UK address? I am personally placing a ~50% penalty on evaluating cost effectiveness (note that I even work on pro-climate change policy research). So I would start to recoup the material losses after ~2 years.

Just as a side note, there are schemes in the UK for the elderly and vulnerable that will come and fit radiator reflectors (for free) to radiators that are on outside facing walls (under windows) to help reduce the costs, so they seem like they do actually have some purpose.
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cchanaApr 11 '12 at 15:01

These charts are deceptive. The U value is inverse of the more commonly seen R-value. What wall has a U of 2.1? A single pane of glass has a U of 1.2. An insulated wall has a U more like 0.04. The energy lost into the wall behind the radiators is minuscule in comparison to the energy lost through the total building envelope, except maybe if your walls are not insulated at all.
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bcworkzJul 14 '12 at 18:35

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A solid victorian brick wall (approx 215mm) has a U-value of about 2.1 W/m². A single pane of glass has a centre U-value of around 5 W/m². An insulated wall will have a U-value of between 0.25 and 0.15 W/m² depending on construction and insulation used. Loft insulation roll will have a thermal conductivity of around 0.04 W/mk.
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user15051Sep 12 '13 at 8:27